Immediately after learning his girlfriend of three years, Kristen Stewart, had cheated on him with her married Snow White and the Huntsman director Rupert Sanders, Pattinson asked his friend and Water for Elephants costar if he could crash at her tranquil estate in Ojai, Calif.

Pattinson was utterly blindsided by the news – which a distraught Stewart delivered just before the story broke July 24 on PEOPLE.com – sources tell PEOPLE in this week's cover story.

"There was no sign that either of them was unhappy," says a source close to the 26-year-old actor. "He would have done anything for her."

While the source says Pattinson has had his "heart ripped out," a Stewart insider says the Twilight heroine, 22, is likewise devastated.

"Telling Robert was the worst thing she's ever had to do," says the insider.

For much more on Stewart's "huge mistake" – including her pleading texts and phone calls to Pattinson – and whether the actor will be able to forgive her, pick up this week's PEOPLE, on newsstands Friday